In 2005, Mr. Felt revealed that he was the one who had secretly supplied Bob Woodward of The Washington Post with crucial leads in the Watergate affair in the early 1970s. His decision to unmask himself, in an article in Vanity Fair, ended a guessing game that had gone on for more than 30 years.

The disclosure even surprised Mr. Woodward and his partner on the Watergate story, Carl Bernstein. They had kept their promise not to reveal his identity until after his death. Indeed, Mr. Woodward was so scrupulous about shielding Mr. Felt that he did not introduce him to Mr. Bernstein until this year, 36 years after they cracked the scandal. The three met for two hours one afternoon last month in Santa Rosa, where Mr. Felt had retired. The reporters likened it to a family reunion.

Mr. Felt played a dual role in the fall of Nixon. As a secret informant, he kept the story alive in the press. As associate director of the Federal Bureau of Investigation, he fought the president’s efforts to obstruct the F.B.I.’s investigation of the Watergate break-in.

Without Mr. Felt, there might not have been a Watergate — shorthand for the revealed abuses of presidential powers in the Nixon White House, including illegal wiretapping, burglaries and money laundering. Americans might never have seen a president as a criminal conspirator, or reporters as cultural heroes, or anonymous sources like Mr. Felt as a necessary if undesired tool in the pursuit of truth.

Like Nixon, Mr. Felt authorized illegal break-ins in the name of national security and then received the absolution of a presidential pardon. Their lives were intertwined in ways only they and a few others knew.

Nixon cursed his name when he learned early on that Mr. Felt was providing aid to the enemy in the wars of Watergate. The conversation was recorded in the Oval Office and later made public.